An important component of disposable absorbent articles such as diapers is an absorbent core structure comprising super-absorbent polymers, or SAP's, which ensure that large amounts of bodily fluids, e.g. urine, can be absorbed by the article during its use. When the SAP's were first introduced in absorbent articles, a significant decrease in the article's thickness was achieved, because a much smaller volume of super-absorbent polymer was needed, compared to the large volumes of absorbent pulp, traditionally used in absorbent articles. See for example, U.S. Pat. No. 3,699,103 (Harper et al.), issued Jun. 13, 1972, and U.S. Pat. No. 3,770,731 (Harmon), issued Jun. 20, 1972, that disclose the use of such absorbent polymers in absorbent articles and see for example, U.S. Pat. No. 4,673,402 (Weisman et al.), issued Jun. 16, 1987 and U.S. Pat. No. 4,935,022 (Lash et al.), issued Jun. 19, 1990, that disclose dual-layer core structures comprising a fibrous matrix and absorbent polymers useful in fashioning thin, compact, non-bulky diapers.
These absorbent polymers are often made by initially polymerizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g., sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like in the presence of relatively small amounts of di- or poly-functional monomers such as N,N′-methylenebisacrylamide, trimethylolpropane triacrylate, ethylene glycol di(meth)acrylate, or trialkylamine. The di- or poly-functional monomer materials serve to lightly cross-link the polymer chains thereby rendering them water-insoluble, yet water-swell able. These lightly cross-linked absorbent polymers contain a multiplicity of carboxyl groups attached to the polymer backbone. These carboxyl groups generate an osmotic driving force for the absorption of body fluids by the cross linked polymer network.
Absorbent polymers useful as absorbents in absorbent members and articles such as disposable diapers need to have adequately high sorption capacity, as well as adequately high gel strength. Sorption capacity needs to be sufficiently high to enable the absorbent polymer to absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article. Gel strength relates to the tendency of the swollen polymer particles to deform under an applied stress, and needs to be such that the particles do not deform and fill the capillary void spaces in the absorbent member or article to an unacceptable degree, so-called gel blocking, thereby inhibiting the rate of fluid uptake or the fluid distribution by the member or article. (Once gel-blocking occurs, it can substantially impede the distribution of fluids to relatively dry zones or regions in the absorbent member or article and leakage from the absorbent article can take place well before the particles of absorbent polymer in the absorbent article are fully saturated or before the fluid can diffuse or wick past the “blocking” particles into the rest of the absorbent article.)
In general, the permeability of a zone or layer comprising swollen absorbent polymer can be increased by increasing the cross link density of the polymer gel, thereby increasing the gel strength. However, this typically also reduces the absorbent capacity of the gel undesirably. See, for example, U.S. Pat. No. 4,654,039 (Brandt et al.), issued Mar. 31, 1987 (reissued Apr. 19, 1988 as U.S. Reissue Pat. 32,649) and U.S. Pat. No. 4,834,735 (Alemany et al.), issued May 30, 1989.
In the past decade, significant investments have been made to improve the performance of such SAP's, e.g. to provide a higher absorbent capacity per volume, to improve fluid distribution throughout the SAP's, to reduce so-called gel blocking of the SAP's.
At relatively high concentrations, an important property of these absorbent polymers is their permeability/flow conductivity. The permeability/flow conductivity of a material can be defined in terms of its Saline Flow Conductivity (SFC). SFC is a measure of the ability of a material to transport saline fluids, such as the ability of a layer comprising swollen absorbent polymer to transport body fluids. Typically, an air-laid web of pulp fibers (e.g., having a density of 0.15 g/cc) will exhibit an SFC value of about 200×10−7 cm3·sec/g.
Absorbent polymers with relatively high permeability (SFC values) can be made by increasing the level of cross linking, which increases the strength of the swollen gel, but this typically also reduces the absorbent capacity of the gel undesirably, as described above. The key focus has so far been to modify the surface cross-linking such that an optimum SFC value and absorbent capacity are achieved at the same time, e.g. such that one does not compromise the other too much.
In addition to surface cross linking, several other approaches have been utilized to improve the permeability, and/or the absorption rate of the absorbent. For example, U.S. Pat. No. 5,419,956 (Roe), issued on May 30, 1995, describes the addition of inorganic particles such as silica to super-absorbent polymers to improve fluid uptake rate and to increase fluid distribution. U.S. Pat. No. 4,454,055 (Richman) describes the addition (either under wet or dry conditions) of between 1 and 75% of an extender material such as cellulose derivatives, or inorganic materials like clay or minerals. The objective of this development is an increase of absorbent capacity. It should also be noted that silica and certain inorganic powders and also wax have been used as process aids when producing super absorbent polymer particles, namely as anti caking agents, see for example U.S. Pat. No. 6,124,391. The inorganic material or the wax is then typically not present in the form of a real coating, although the super absorbent polymers may be partially coated with this material.
However, there is still a need to improve the absorbent capacity and gel strength at the same time.
Also there is still a need to provide even thinner absorbent articles, e.g. sanitary napkins and diapers. One way of doing this is to reduce the amount of fibers used in the absorbent cores of the articles, or even not to incorporate any fibers at all in the absorbent cores, but only super absorbent polymers. As described above, reducing the fiber content between the super-absorbent polymers increases the risk of gel blocking. Thus, there is a need to provide thinner cores with out much or any fibers, which do not suffer from gel blocking.
The inventors have now found an improved super absorbent material that provides higher SFC values and higher gel strength and that does not suffer from gel blocking and that even can be used in fiber-free absorbent structures.
They found that it is important to ensure that the super absorbent material provides high capillary forces. The inventors found that it is thereto important that the super-absorbent material has a specific low contact angle, correlated with a specific surface energy and that the liquid to be absorbed has to remain a specific surface tension, e.g. should not reduce too much in surface tension. It is believed that this has so far not been recognized in the art. In fact, it has been found that the coating used in the prior art, such as hydrophilic silica, may negatively affects the contact angle and the related surface energy of the super absorbent material and therefore this type of coated super absorbent material does not provide the required capillary forces. Equally, surfactants, which are also used in super absorbent materials, can significantly reduce the capillary forces due to the effect they have on the surface tension of the liquid and the surface energy of the material.
The inventors found that certain specific coatings can provide the required surface energy of the super absorbent material, whilst not negatively affecting the surface tension of the liquid to be absorbed. They found that the coating has to comprise a specific organic coating material with one or more polar groups, which typically is relatively small as not to change the liquid's surface tension too much, even when the coating material is water-soluble.
They found that absorbent structures or articles comprising such coated super absorbent polymers not only provide excellent SFC and gel strength values at the same time, but that such absorbent structures can also be fiber-free, and hence thinner, without the risk of gel blocking.
In addition, the absorbent structures or articles have higher capillary forces leading to a better drainage of the layer(s) closest to the wearer's skin, and to a better liquid spreading or wicking over the length and width of the absorbent structure.